<header
    class="site-header
    <?php
        $header = get_field( 'header',$post->ID );
        if ($header) {
            echo esc_attr( $header );
        }
        else {
            echo 'white';
        }
    ?>"
>

<header class="site-header <br /> <b>Notice</b>: Trying to get property of non-object in <b>/Applications/MAMP/htdocs/theme/wp-content/themes/theme/header.php</b> on line <b>28</b><br /> white">
</header>

<header id="masthead" class="site-header

    <?php

        if ( is_404() ) {
            echo 'white';
        }

        else {

            $header = get_field( 'header',$post->ID );

            if ( $header ) {
                echo esc_attr( $header );

            }

            else {
                echo 'white';
            }
        }
    ?>"

role="banner">